[macOS] IME preedit lost on focus change

[hyunq21]

What does the pull request do?

Fixes two related macOS Korean IME issues where in-progress composition
(preedit) text was lost when focus moved away from a TextBox while
composing — e.g. while typing Korean and then clicking another control or
switching focus between text boxes.

What is the current behavior?

On macOS, when a Korean IME composition is active (marked/preedit text is
shown) and the user moves focus away — by clicking another control or
focusing a different TextBox — the composing text is silently discarded
instead of being committed. The half-composed characters are lost.

This happens because:

• AvnTextInputMethod::Reset() was an empty no-op, so the native input
  context was never told to finalize/clear its marked text when the
  Avalonia input client changed.
• A mouse press during an active composition was not forwarded to the
  NSTextInputContext, so the IME never got the chance to commit the
  composition before the click was handled.

What is the updated/expected behavior with this PR?

The active composition is committed (not dropped) when focus changes:

• Clicking another control while composing commits the current preedit.
• Switching focus between text boxes commits the preedit and clears the
  stale composition state.

To test:

1. Run on macOS with the Korean IME (2-Set Hangul).
2. Start composing text in a TextBox (leave it mid-composition, with the
   underlined preedit visible).
3. Click another control / another TextBox.
4. The composed text is now committed into the original TextBox instead
   of being lost.

How was the solution implemented (if it's not obvious)?

• AvnView: on a mouse button-down while hasMarkedText is true, forward
  the event to [self inputContext] handleEvent: so the IME commits the
  in-progress composition before the click is processed.
• AvnTextInputMethod::Reset() now calls a new resetInputMethod delegate
  method on the view, which clears the preedit on the Avalonia client,
  resets the marked range, and calls discardMarkedText on the input
  context so no stale composition state leaks across focus changes.
• Added the resetInputMethod declaration to AvnTextInputMethodDelegate.

Checklist

• Added unit tests (if possible)? — N/A, native macOS IME behavior is
  not covered by the automated test harness.
• Added XML documentation to any related classes? — N/A, native code only.
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #19964
Fixes #15183

[hyunq21]

2026-06-17.3.42.19.mov

[cla-avalonia]

• All contributors have signed the CLA.

[hyunq21]

@cla-avalonia agree

[Copilot]

Pull request overview

Fixes macOS Korean IME composition (preedit) loss/misplacement when focus changes by ensuring the native NSTextInputContext gets a chance to commit an active composition on mouse-down, and by explicitly resetting native/managed preedit state on input-client changes.

Changes:

• Forward mouse button-down events to NSTextInputContext when there is active marked text so the IME can commit before Avalonia processes the click.
• Implement AvnTextInputMethod::Reset() to invoke a new delegate hook that clears preedit/marked state and discards marked text in the native input context.
• Extend AvnTextInputMethodDelegate with resetInputMethod and implement it on AvnView.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
native/Avalonia.Native/src/OSX/AvnView.mm	Forwards mouse-down to the input context during composition and adds a reset hook to clear marked/preedit state.
native/Avalonia.Native/src/OSX/AvnTextInputMethodDelegate.h	Extends the delegate protocol with resetInputMethod used during client resets.
native/Avalonia.Native/src/OSX/AvnTextInputMethod.mm	Implements Reset() to call the delegate and trigger native IME state cleanup on focus/client changes.

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0066569-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[hyunq21]

#19636 also appears to be the same issue

[Gillibald]

LGTM

[hyunq21]

I missed something. Korean preedit is always a single character, but Japanese and Chinese preedit can span multiple characters, so we must not commit the preedit when the user clicks inside it. We should only commit it when a point outside the preedit is clicked.

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0066850-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Gillibald]

We do not need to fix this as part of this PR. I have a dedicated PR for a more complete NSTextInputContext implementation that covers this as well. That PR is based on this PR.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067857-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]